[Agda] Unreadable constraints, problem with name printing?

[Andreas Abel]

What does this constraint mean?

  (_138 {.X} {.R} {.S} R⊆S step x {_} {_}
    {_139 {.X} {.R} {.S} R⊆S step x {_} {_}}
    {_140 {.X} {.R} {.S} R⊆S step x {_} {_}}) = _ : .X

I am puzzled about the _ on the r.h.s., and also in the implicit 
arguments to the meta.

Is this supposed to be a variable name?  Are all _ the same variable?

Cheers,
Andreas

-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Theoretical Computer Science, University of Munich
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andreas.abel at ifi.lmu.de
http://www2.tcs.ifi.lmu.de/~abel/